This invention relates generally to apparatus for mounting and fastening printed circuit boards or cards within a rack or chassis, and, more particularly, to apparatus of the kind that includes an elongated wedge assembly that expands transversely to fasten the board in place.
Elongated wedge assemblies of this particular kind have been in common use for fastening printed circuit (i.e., PC) boards within an elongted slot in a rack or chassis. The assembly typically includes a center wedge having sloped surfaces at its opposite ends and two end wedges having sloped surfaces that abut against the center wedge's sloped surfaces. A screw extends lengthwise through the center wedge and connects together the two end wedges. The end wedge remote from the head of the screw has a threaded bore engaged by the screw such that a clockwise rotation of the screw draws the two end wedges toward each other, causing them to deflect transversely on the sloped abutting surfaces. This increases the assembly's effective width and thereby fastens the PC board tightly within the slot. Subsequently rotating the screw in a counterclockwise direction moves the two end wedges apart from each other, to bring them back into longitudinal alignment with the center wedge and, thereby, to release the clamping force on the PC board.
It is desirable to provide a means for the remote end wedge when the wedge assembly is loosened. Excessive unthreading can allow one or both of the end wedges to rotate relative to center wedge and thereby prevent an insertion of the assembly into the chassis slot. It also can allow the remote end wedge to become fully disengaged from the assembly.
One technique for preventing an excessive unthreading of the screw is to insert a locking pin through a hole drilled diametrically through the screw's remote end. With the pin in place, the screw can be unthreaded only to the point where the pin first engages the remote end wedge. This technique, though effective, is not entirely satisfactory. It is generally quite difficult to drill a hole diametrically through a screw, particularly if the screw has a small diameter. Also, it is necessary to remove any resulting drill burr on the thread so that the remote wedge can be threaded onto the screw.
Another technique attempted in the past for preventing the screw from being inadvertently unthreaded from the remote end wedge when the assembly is loosened is to thread a friction nut onto the screw's remote end. When the assembly is being tightened, the friction nut simply rotates along with the screw and remains spaced from the end wedge. When the assembly is being loosened, the nut again rotates along with the screw, until the screw unthreads to the point where the nut contacts the end wedge. When that point is reached, the hope is that the nut will bear against the end wedge and prevent further unthreading. However, if the frictional torque of the nut bearing against the wedge exceeds the frictional hreads, as is usually the case, then the screw can continue to unthread. This has occurred in the past even where a washer or spacer has been interposed
end wedge.
It should, therefore, be appreciated that there is a need for a locking wedge assembly that is configured to effectively prevent an excessive unthreading of the screw from the remote end wedge when the assembly is loosened and that is inexpensive and convenient to manufacture and assemble. The present invention fulfills this need.